Nuclear magnetic resonance (NMR) chemical shift spectroscopy has been in use for a relatively long time. For example, in 1950 E. L. Hahn published an article in the Physical Review, Volume 80, pp 580 which disclosed a sequence to obtain stimulated echo signals (STE) for use in spectroscopic experiments. In 1973 P. C. Lauterbur in an article published in Nature (London) 242, 89/90 disclosed the use of field gradients for determining the source location of free induction decay (FID) signals obtained in NMR experiments. The knowledge of the source of the FID signals enables the MR acquired data to be used to reconstruct interior images of the subject placed in a strong magnetic field.
It has long been known that when atomic nuclei that have net magnetic moments are placed in a strong static magnetic field, the nuclei ("spins") precess about the axis of the field at the Larmor frequency given by the equation: EQU f=.gamma.Bo/2.pi.
in which:
.gamma. is a gyromagnetic ratio, constant for each NMR isotope which exhibits a net magnetic moment; PA1 Bo is the strength of the magnetic field; and PA1 .pi. is the well known constant 3.1416+. PA1 Ta=TR.n.sub.x.n.sub.y (assuming phase encoding along the X and Y axes), PA1 .DELTA.f is an offset frequency (added to the Larmor frequency); and PA1 .DELTA.F is the bandwidth of the RF pulse.
As is well known magnetic resonance imaging (MRI) uses a relatively strong static magnetic field having a given direction which is aligned with the Z axis of a cartesian coordinate system. The strong static magnetic field causes the nuclei of certain elements such as hydrogen to align with the field. Subsequently radio frequency pulses of sufficient amplitude and/or time duration are applied to perturb or nutate the aligned nuclei. The rotational frequency of the RF precession and the frequency of the RF pulse is the above noted Larmor frequency.
After the termination of the RF pulse the rotated nuclei or spins tend to realign with the static magnetic field. The precession of the transverse component in the magnetic field generates RF signals also having a Larmor frequency. These signals are known as free induction decay (FID) signals. It is these signals that are received to provide information on the spin density of the element whose spins have been rotated by the RF pulse.
There are many different methods used for obtaining the FID signals. Among the methods and probably one of the most popular methods at the present time is the spin echo method. This method is well known and will not be elaborated on herein.
In imaging, in general, the scientists are always endeavoring to increase the spatial resolution and lower the time required to provide the image. These are contrary aims; that is decreasing the time generally may require decreasing the resolution and generally will adversely effect the signal to noise ratio. Thus, a method for decreasing the time while maintaining the same resolution an/or signal to noise ratio or a method for increasing the resolution while imaging during the same time period is highly desirable. In MR imaging, increasing the time of acquiring an image does not pose any known danger to the patient because there is no dangerous radiation being used; nonetheless, since patient comfort and throughput are important considerations effecting both the picture quality and the economics of the system, clincians and imaging scientists are always interested in decreasing the time required for acquiring images. In some cases the time saved might be used for accumulating several images of the same slice and subsequently averaging the several images to improve the signal-to-noise ratio.
A further goal desired by imaging scientists is to be able to zoom during the acquisition stage. In other words, during the imaging process if a particular portion of the body shows an interesting manifestation; it is often desirable to zoom in on this manifestation and to thereby focus on the manifestation to the exclusion of other data. This is presently generally accomplished to MRI systems as a computer step after the acquisition of the data, especially if the imaging is to be accomplished within a given time frame. However, no increase of the spatial resolution can be achieved by such manipulation of the data. It would be desirable to be able to zoom during the acquisition of data. Such zooming would increase the resolution of the portion of the image focused upon in a natural manner.
A prior art problem encountered when zooming during the acquisition of data is that "aliasing" artifacts caused by undersampling may be generated unless the number of encoding cycles is increased with a proportional increase of the total acquisition time. The relationships between the field of view, the resolution and the data acquisition time are shown as follows:
The size of the volumetric aquisition matrix is: EQU n.sub.x.n.sub.y.n.sub.z
where n.sub.x, n.sub.y and n.sub.z denote the size of the matrix along the X, Y and Z axis, respectively.
The volume of a voxel is EQU V=1.sub.x.1.sub.y.1.sub.z
where 1.sub.x, 1.sub.y and 1.sub.z are the dimension along the X, Y and Z axis, respectively.
The field of view FOV is FOV;=1i*ni where i=x, y, z.
The resolution L; at voxel n; is: EQU L;=n;/FOV;
The data acquisition time Ta is:
where TR is the repetition time
It is apparent that restricting the FOV increases the resolution with a fixed acquisition matrix. Similarily restricting the FOV with a fixed resolution will decrease the acquisition time.
Localization of the volume of interest is important for medical diagnostic application of MRI. Selection of a cubic volume can be achieved by application of RF pulse sequences comprising three consecutive tailored RF pulses, each in the presence of a different one of the three orthogonal gradients. The use of such pulse sequences such as 90 degrees, 180 degrees and 180 degrees has been reported by R. E. Gordon and R. J. Ordidge, in a report entitled "Volume Selection for High Resolution NMR Studes" in the Proceedings of the SMRM Third Annual Meeting, 1984 at pp 272 et seq. A pulse sequence using a composite pulse such as selective 45 degrees, non-selective 90 degrees and selective 45 degrees with the composite pulse applied three times has been reported in an article by W. P. Aue, S. Muller et al in the Journal of Magnetic Resonance, vol 56 pp 350 et seq. "A Selective Volume Method for Performing Localized NMR Spectroscopy", is the subject of the U.S. Pat. No. 4,480,228 which was issued on Oct. 30, 1984.
The 90-180-180 prior art pulse sequence procedure for spatially localizing the NMR signals received yields signals that are strongly dependent on the T2 relaxation times of the spins that provide the signals. This dependance on the T2 relaxation times makes it difficult to detect signals with short T2 relaxation times.
Another problem with the prior art pulse sequence methods for spatially localizing the acquired signals is that the RF power transmitted to acquire data tends to heat the tissue of the subject. It is therefore incumbent on the designers of such methods to minimize the RF power deposition.
Yet another problem caused by the employment of 180 degree RF pulses to spatially localize time acquired signals is that there is some loss of definition in the selected volume.
The ability to obtain stimulated echoes as previously noted has been known to those skilled in the art for a long time. It is also known that among the benefits obtained by using stimulated echoes, in NMR imaging for example, is that no 180 degree pulses are needed. The 180 degree pulses require more power than 90 degree pulses, therefore, when acquiring data using stimulated echoes, the applied power is considerably reduced as compared to the spin echo data acquisition sequences where 180 degree pulses are used.
In an article entitled "Stimulated Echo Imaging" by J. Frahm, et al which appeared in the Journal of Magnetic Resonance, Vol. 64, pp 81-93, (1985) it was noted that stimulated echo imaging reduce dependence on T2 relaxation time and that the RF power requirements are reduced. It is well known that the stimulated echo procedure gives rise to a number of unwanted FIDs and echoes and until now nobody has applied stimulated echo pulse sequences for acquiring imaging data of spatially localized volumes.